Thin Lenses A lens wouldn’t be much good if the only function it had was to focus or spread parallel rays coming from a near-infinite distance. Fortunately, lenses perform interesting operations on rays under other conditions, too. For instance, a converging lens can take the rays bouncing off an object and project them as a real image onto a surface somewhere on the far side of the lens. There is a relationship between the object distance ( l ) and the focal length of the lens (which is usually simply given) that allows one to predict the image distance ( l ’ ) where a crisp, focused image will form. It is called the thin-lens equation , and is most often written 1/ l + 1/ l ’ = 1/ f . You may find this next form more useful though, since the image distance is usually what is sought: l ’ = l f / ( l – f ) . There are certain assumptions built into this equation of which you should be aware. First, it is assumed that the distances are all measured from the center of the lens. This creates a slight error since even the thinnest of lenses has some
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This note was uploaded on 04/05/2012 for the course PHYS 131 taught by Professor Tibbets during the Spring '11 term at Cuyamaca College.